tD-NMR and chemometrics
for fat analysis in food
In food science, fat content determination is usually performed with standard methods of wet
chemical analysis. Such analyses sometimes use toxic organic solvents and consequently generate
toxic waste. They are also slow and destructive, and cannot be applied to every product.
Time-domain nuclear magnetic resonance (TD-NMR) was recently
demonstrated to be a promising non-invasive technique for measuring fat
content through packaging—with almost no limitations on sample size, and
without weighing the samples. Products can be measured one-by-one,
promoting accurate quality control for final consumers [ 1].
TD-NMR spectrometers have been used to measure packaged mayonnaise, salad dressing, nuts, and raw beef. In this article, we will show
some results that describe the use of the TD-NMR signals profile and the
chemometrics approach to developing models that predict fat content.
UsINg tHE tD-NMR sIgNAl
The NMR phenomenon is observed when a nucleus with an intrinsic angular momentum, or spin, and an associated magnetic moment are placed in an
external magnetic field. For samples containing nuclei with spin ½, such as 1H,
19F, the magnetic field separated the spins into two energy levels. The TD-NMR
signal is observed when a sample is irradiated with electromagnetic radiation
in the radio frequency region (between 2 and 80 MHz) that matches the energy
difference between the two spin levels. After the irradiation, the NMR signal
decays as a function of time; this is known as free induction decay, or FID. The
NMR signal can also be manipulated by several pulses to measure several NMR
parameters, such as the longitudinal (T1) and transverse (T2) relaxation times.
The sequence developed by Carr-Purcell and Meiboom-Gill (CPMG) to measure
T2 has been widely used to measure fat content in food [ 2].
The determination of fat content is based on the fact that transverse relaxation time (T2) of 1H depends on the effect of molecular motion in the homo-nuclear dipolar interaction. In the case of fats and other macromolecules, T2 is
known to correlate, in macroscopic materials, with such properties as viscosity, melting temperature, and crystallinity. Thus, T2 obtained with CPMG pulse
sequence can be used for the non-invasive characterization of fat content and
fat quality in food products.